Contributors PJM participated in the design of the study, data analysis and carried out the manuscript drafting. TILN performed the data
analysis, participated in the design of the study and helped to draft the manuscript. AH participated in the design of the
study, data analysis and helped to draft the manuscript. All authors have read and approved the final manuscript.

Accepted 20 March 2012

Published Online First 17 April 2012

Abstract

Background Mechanical joint stress imposed by high body mass index (BMI) is associated with increased risk of knee and hip osteoarthritis.
This prospective study investigated the independent and joint association of BMI and physical exercise on risk of knee and
hip osteoarthritis.

Methods The study includes 15 191 women and 14 766 men in the Norwegian HUNT Study without pain or physical impairment at baseline.
Occurrence of self-reported physician-diagnosed osteoarthritis was assessed at 11 years of follow-up.

Results BMI was positively related to risk of knee osteoarthritis (Ptrend<0.001), with an RR of 4.37 (95% CI 3.01 to 6.33) in women and 2.78 (95% CI 1.59 to 4.84) in men, comparing obese and normal
weight persons. No clear association was observed for hip osteoarthritis. Obesity increased the risk of severe activity-limiting
osteoarthritis, with an RR of 2.30 (95% CI 1.68 to 3.15) and 2.50 (95% CI 1.56 to 4.03) in women and men, respectively. Physical
exercise did not modify the above associations (Pinteraction>0.34). Exercise intensity was not associated with risk of osteoarthritis in any BMI category; that is, obese persons reporting
high-intensity exercise had an RR of 1.28 (95% CI 0.59 to 2.79) for severe osteoarthritis compared with inactive persons.

Conclusion High BMI increases the risk of knee osteoarthritis and severe osteoarthritis. Physical exercise does not increase the risk
of osteoarthritis at any level of BMI, suggesting that exercise could be encouraged also among individuals with excessive
body mass, without concern for an increased risk of osteoarthritis.